Soil Water3 Phases
Solid
Liquid
Gas
Bent Molecule
Neutral, but POLAR
Polarity controls movement
and retention in soil
Forms of Water Potential – Ψ
ΨG Gravitational Potential (+ value)
- determined by the height of water above a reference point
- water flows downward under gravity
ΨM Matric Potential (- value)
- determined by the strength of the attraction of water to the soil matrix
- most important for unsaturated flow (dry soils)
- water flows from zones of wet soil to zones of dry soil (less negative more negative)
ΨS Solute (Osmotic) Potential (- value)
- determined by the concentration of solutes in the soil water
- more negative for high solute concentrations
- water flows from zones of low solute to high solute concentrations
- most important for root-water interaction
ΨP Pressure Potential (+ or – value)
- determined by amount of pressure imposed on soil water
- usually zero, but there are exceptions
Flooded Soils
Soils with water tables – ie artesian wells
Infiltration Rate ~ cm/hr
in general
Infiltration rate = hydraulic conductivity x
Hydraulic Conductivity is:
a measure of characteristics unique to each soil type which affects water flow
it is a function of:
1) soil water content
2) size distribution and tortuosity of pores
∆Ψ
∆Χ
Saturated Flow- occurs when pores are water-filled
- most rapid in larger pores (e.g. sands)
- flow in response to potential gradients:
Gravity (ΨG) & Pressure (ΨP)
- water drains freely from large pores by gravity and pressure Ψ’s
Unsaturated Flow- more common that saturated flow
- more important than saturated flow?
- controlled by Matric Potential (ΨM)
- movement from areas of less negative to more negative potential
Capillary Rise
Capillarity is the tendency for water to rise against the gravity
Function of Water Adhesion and Cohesion
Figure 5.6
Water Movement vs. Water Storage
The soil matrix retains water against the pull of gravity
- this is controlled by the matric potential (ΨM)
- and is a function of
Capillarity – important in wet soils
Adsorption – important when soil is below field capacity
Solute Potential (ΨS)
- force that acts across a membrane – ex. root cell membrane
- due to hydration of ions
- important in saline soils
Effect of is to reduce the free energy
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